Fiber optic lasers continue to proliferate throughout a significant number of places in the world including but not limited to deep sea installations, on board high speed aircraft, and interstellar spacecraft. As the applications become more and more demanding, these fiber lasers are required to withstand significantly harsher environments, including extreme temperatures and intense vibrations on rotary wing military aircraft. Designing these fiber lasers to withstand these environments ensures guaranteed reliable performance.
The current practice of packaging fiber optics includes housing the active fibers of the fiber laser on the outside of spools and connecting these active fibers to passive fibers in passive service loops. The passive service loops often contain fiber from many different parts of a laser and contain varying intensities of light, which are generally different wavelengths. Regardless of failures in the active doped fiber, a failure in a single service loop may compromise or destroy other parts of the fiber laser and can result in catastrophic laser failure due to the spatial intimacy of those fibers. More importantly, failures occur not only due to mechanical breakage from vibration or pinching of the optical fibers, but also from thermal stress due to the large amounts of heat generated in the active fibers. For instance, the presence of kinks or sharp radiuses in the active fibers can create hotspots which are subject to failure, especially in high power laser operation. Moreover, since passive fibers are connected to the active fibers at a splice, any discontinuities at the splice create a source of heat. Additionally, fiber Bragg gratings are typically located at either end of the active doped fiber and these too must be temperature controlled. The result of not dissipating the heat at these hotspots is failure.
By way of further background, fiber transitions from one portion of a fiber laser to another are often left free-floating, e.g., without making contact with a mounting surface. This free-floating fiber is vulnerable to high loads and strenuous vibration modes due to the suspended nature of the fiber. Common strain/stress reliefs using external flexible boots ease the transition from a component to a stabilizing surface, but these devices do not eliminate the potential for failure at that interface. Often times, strain reliefs are clamped down so hard that they squeeze the core of the optical fiber which alters the light transmission through these fibers.
Fiber optic lasers usually involve packaging a symmetrical cylindrical package such as a spool adapted to carry a length of doped active fiber wound around the barrel of the spool. Optical energy is injected into the active fiber which, in-turn, produces the desired laser output. Typically the optical fibers for such lasers are relatively long, generally on the order of between ¼ m and 10 m in length depending on the dopants utilized. In terms of providing sufficient mechanical support for the fiber and in terms of thermal management, such long lengths of fiber may present issues when the fiber lasers are packaged in compact units. If a spool is used to house the fibers, a few meters of active fiber may typically require, in one example, seven turns on the spool. Typically, for spool-mounted optical fiber, there are terminations at either end of the active doped fiber which are subject to failure. These terminations may utilize various rare earth elements, such as thulium, holmium, ytterbium, erbium, and other rare earth elements, to dope the glass fibers. It is at these terminations that passive fibers or fiber Bragg gratings are connected. As mentioned above, these failures can be both mechanical and thermal failures.
With respect to high output power, and more particularly with respect to the utilization of these lasers in directed energy applications, it is important to be able to combine the output of the lasers to produce a combined output beam that can exceed 1000 W. It may, therefore, be necessary to combine the fiber laser outputs of, for instance, a number of 100 Watt lasers to obtain the full output power. Additionally, if modulators or other apparatus at the ends of or along the fiber optic lengths are used, the mechanical and thermal aspects of connecting these units to the fibers should be controlled. The wiring in proximity to the fibers must also be controlled, such that the wiring does not touch the fibers or apply abrasive forces to them in any way when the unit experiences vibrations and other environmental factors.
Conventional fiber packaging strategies utilize plastic fiber-routing clips and other hardware which are not adequate for high-stress embodiments, such as those involving military vehicles under extended periods of operation. These conventional fiber routing components are too flexible, they do not retain the fiber in place adequately, they cannot withstand extreme temperatures and vibration, and they are often designed too generically to satisfy intensive size, weight and power (SWaP) demands. Typical conventional fiber-routing hardware and strategies require significant modification to survive demanding environments.
Furthermore, in conventional low-power fiber laser systems, there is no thermal management, even though the thermal characteristics of laser systems affect their performance. The major heat generating sources are the laser diodes and the doped fibers themselves. Thus, in high power fiber optic laser applications, there is a considerable amount of heat generated within the fiber itself or at the connections to the fiber that must be dissipated. Conventional packaging techniques often utilize concentric packaging techniques in which various sections of the active fibers are coiled, one inside the other, in a concentric fashion. However, in such concentric packaging applications, there are heat-generating elements. When fiber optic coils are stacked inside one another in a radial stacking pattern, there is insufficient thermal management.
Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.